This invention relates to a method of chemical logging of wells drilled in geothermal formations. More specifically, this invention relates to a method of maintaining and monitoring a chemical log during a well drilling operation to detect the presence of geothermal aquifers before the aquifer is penetrated by the drill.
Growing concern over shortages of fossil fuel has led to increased emphasis on finding and developing new sources of thermal energy such as that which is available in certain geological formations throughout the world. These formations, called geothermal reservoirs, contain usable quantities of warm or hot water in underground aquifers reasonably near enough to the earth's surface to be recoverable. These geothermal aquifers can provide energy for driving electrical generators, for space heating and for performing other energy functions. The growth of the geothermal industry has created a need for techniques which can be used, during drilling operations, to assist in the determination of well depth, casing location and well development.
When drilling wells in geothermal formations, it is usually desirable to be able to detect the presence of a warm or hot water aquifers before the aquifer has been penetrated by the drill bit for several reasons. It permits the setting of well casing in place in the borehole in time to prevent possible contamination of any fresh water aquifers already penetrated by the drill string with upwelling geothermal water which often contains undesirable minerals such as large quantities of sodium chloride or low levels of fluorine on sulfur. Wells are drilled using either a mud or water, as a drilling fluid. Mud is preferred since it reduces the amount of water which must otherwise be used, seals off porous areas in the earth to prevent loss of water, lubricates the drill bit and flushes rock cuttings from the borehole better than water. However, penetration of a potentially productive geothermal aquifer by the drill bit while using mud as the drilling fluid could result in the aquifer being wholly or partially sealed by contact of the mud on the warm porous rock formation. This could result in the loss of the aquifer as source of geothermal energy. Thus, knowledge of the existence of an aquifer in time to permit discontinuance of the use of mud, could prevent loss of a productive aquifer.
A chemical log of a well is a profile of the concentration of chemical elements found in the various geological formation fluids relative to the depth at which they were found. This profile may include elements such as Cl.sup.31 , F.sup.-, Na.sup.+, Ca.sup.++, and SiO.sub.2 in addition to conductivity and pH of the water. The log is prepared by analyzing the drill return fluid at predetermined depths, for example every 10 to 20 meters, and plotting the concentrations of the various analyzed chemical species relative to the drill depth at which the concentrations were present. The log is useful for obtaining information as to what type of aquifer the drill bit has penetrated, the relative temperature of the aquifer and the composition of the aquifer water. However, up until now, the log has not been useful for determining the existence of a geothermal aquifer before it has been penetrated by the drill bit.